The invention is in the field of aircraft shock struts for slowing movement of an aircraft structure toward the ground, particularly during landing or take-off.
Shock struts are a common and necessary component in most aircraft landing gear assemblies. Such shock struts generally utilize an "air cover oil" arrangement wherein a trapped volume of gas is compressed as the shock strut is compressed, and a volume of liquid (usually oil) is metered through an orifice. The air provides suspension and spring rate, and the oil provides a fluid amplified force under dynamic conditions. Some shock struts have a fluid amplified force that depends on the rate at which the shock strut is compressed, of which the well known tapered rod within a fixed orifice is an example (also known as a "metering pin"). Other shock struts provide a fluid amplified force that is generally not dependent on the rate at which the shock strut is compressed. The invention relates to this type of shock strut. An example of a prior art aircraft shock strut having this characteristic is presented in FIG. 1 of the drawings.
FIG. 1 presents a simplified schematic cross-section, not to scale, of prior art aircraft shock strut 100, shown mounted to an aircraft structure 10 by an attachment structure 12. A wheel and tire assembly 14 is attached to the shock strut 100. The aircraft structure 10, attachment structure 12, and wheel and tire assembly are shown in phantom, and structures such as locking mechanisms and retracting mechanisms are not shown in FIG. 1 in order to avoid obscuring shock strut 100. Various arrangements of such structures are very well known in the art, and are not critical in describing shock strut 100.
Still referring to FIG. 1, shock strut 100 is generally cylindrical, having an elongate piston structure 102, and a hollow elongate cylinder structure 104. The cylinder structure 104 receives the piston structure 102 in a manner that permits relative telescoping movement between the cylinder structure 104 and the piston structure 102. The piston structure 102 and the cylinder structure 104 define a sealed elongate cavity 106 at least partially filled with a liquid (shown as dashed lines). A portion 111 of cavity 106 is filled with a gas, the shock strut 100 being a member of the well known "air over oil" class of shock struts. Various sealing arrangements between piston structure 102 and 104 that allow telescoping movement between piston structure 102 and 104 while maintaining a seal are well known in the art, and need not be repeated here. A piston head 108 is attached to the piston structure via a support tube 114 that divides the elongate cavity 106 into an upper cavity 110 and a lower cavity 112. Support tube 114 has openings 116 so that all parts of upper cavity 110 are in fluid communication with each other. Compressing shock strut 100 causes piston structure 102 to move into cylinder structure 104 thereby reducing the volume of cavity 106 which compresses the portion 111 filled with gas. The portion 111 filled with gas thereby provides a spring rate. In addition, piston head 108 has at least one fixed orifice (not shown in FIG. 1), and fluid is pumped from lower cavity 112 to upper cavity 110 as the shock strut 100 is compressed, thereby increasing resistance to compression through fluid amplification while simultaneously dissipating compression energy. These features are presented in more detail in FIG. 2.
Referring now to FIG. 2, a detailed view of the piston head 108 is presented. Piston head 108 is hollow, and has one or more openings 124 and 126 so that upper cavity 110 extends inside piston head 108. The piston head 108 has at least one fixed 118 orifice and at least one pressure relief orifice 120. Relative telescoping movement of the cylinder structure 104 and the piston structure 102 toward each other increases pressure in the lower cavity 112 thereby developing a change in pressure across the piston head 108 that causes the liquid to flow from the lower cavity 112 to the upper cavity 110 through the fixed orifice 118. Piston head 108 has circumferential seal 122 that seals the upper cavity 110 from the lower cavity 112 while permitting relative sliding movement between the piston head 108 and the cylinder structure 104. Piston head 108 and circumferential seal 122 define an effective hydraulic area across which the change in pressure acts thereby adding to the overall resistance to compression of shock strut 100. Piston head 108 also dissipates compression energy by metering the liquid through fixed orifice 118 resulting in dissipation of at least part of the work required to telescope the piston structure 102 toward the cylinder structure 104. Part of the work is stored as recoverable spring energy in the portion 111 filled with gas (FIG. 1) which resiliently suspends the aircraft structure 10 while taxiing on the ground, and also allows the piston structure 102 and cylinder structure 104 to return to their original positions after the compression force is removed, i.e, after take-off. Such operation is well known in the art.
Piston head 108 has a valve poppet 128 received within the piston head 108 moveable from a first position wherein the valve poppet 128 covers the pressure relief orifice 120 to a second position that uncovers the pressure relief orifice 120. The valve poppet 128 is shown in the first position in FIG. 2. Pressure relief orifice 120 is uncovered when the valve poppet is in the second position by providing the valve poppet 128 with at least one venting orifice 130 that aligns with the pressure relief orifice when the valve poppet 128 is in the second position. Valve poppet 128 has a cylindrical probe 132 having an upper portion 134 exposed to liquid in the upper cavity 110 and a lower portion 136 exposed to liquid in the lower cavity 112. Openings 131 in valve poppet 128 place the underside of valve poppet 128 in fluid communication with the rest of upper cavity 126. Cylindrical probe 132 is closely fitted to a mating surface on the piston head 108 to provide an effective seal between the upper cavity 110 and lower cavity 112 under dynamic conditions when the piston structure 102 and cylinder structure 104 rapidly move together. Under dynamic conditions, the upper and lower portions 134 and 136, and piston head 108 cooperate to define a hydraulic area across which the change in pressure acts and develops an upward force on the valve poppet 128. With the cylindrical probe 132 the hydraulic area is the circular cross-sectional area of the probe.
Still referring to FIG. 2, a spring 138 urges the valve poppet 128 into the first position with a predetermined force. The change in pressure acting across the lower portion 136 and the valve poppet upper portion 134 develops a force acting against the spring 138 that moves the valve poppet 128 from the first position to the second position upon the force exceeding the predetermined force. Moving the valve poppet 128 to the second position uncovers the pressure relief orifice 120 via venting orifice 130 upon which the change in pressure causes the liquid to flow from the lower cavity 112 to the upper cavity 110 through the pressure relief orifice 120 and venting orifice 130. The valve poppet 128 begins to move to the second position when the pressure across the piston head 108 exceeds a predetermined value due to the fixed orifice 118 being unable to pass enough fluid to keep the change in pressure from exceeding the predetermined value, so the pressure relief orifice 120 opens to provide an additional flow path. Opening the pressure relief orifice 120 decreases the change in pressure across the piston head. The spring 128 returns the valve poppet 128 to the first position upon the force decreasing to less than the predetermined force. The valve poppet may oscillate between the first and second positions in response to loading conditions imposed on the shock strut, thereby maintaining a generally constant predetermined change in pressure across the piston head 108. The valve poppet 128 thereby acts as a regulator. The change in pressure is determined by the predetermined spring force, and the hydraulic area across the probe 132 upon which the change in pressure acts. Thus, the fluid amplification force generated by the shock strut 100 is generally independent of the speed with which the piston structure 102 moves toward the cylinder structure 104. The various orifices are sized based on the worst case, corresponding to the greatest piston head speed and stroking force. The predetermined change in pressure is determined by the needed stroking force in the worst case, and the orifices are designed to provide the predetermined change in pressure, or a little less since valve poppet action actually provides a band within which the change in pressure is maintained, and the center of the band generally corresponds to the predetermined change in pressure.
The spring 138 determines the predetermined force at which the valve poppet 128 begins to move. A helical spring 138 is used which has a spring constant, resulting in some change in the spring force as the valve poppet moves from the first position to the second position. The change in spring force from the first position to the second position is minimized in order to provide a narrow target band for the generally constant predetermined change in pressure across the piston head 108. The change in spring force may be minimized by minimizing the spring constant of helical spring 138.
Though shock strut 100 is certainly safe and effective, it has a tendency to generate an oscillatory suspension load in some situations. For example, a sudden impact on the shock strut induced, for example, by the wheel and tire assembly 14 striking a depression in the runway results in an under damped jouncing motion in the cylinder structure 104. In some aircraft, the motion is not objectionable, and the tire is capable of absorbing and mitigating the motion until it damps out due to damping within the tire and low levels of damping inherent in shock strut 100. In some aircraft, however, the motion is objectionable, and may render shock strut 100 unacceptable. In addition, the tendency to generate an oscillatory suspension load renders shock strut 100 unacceptable for certain other applications. For example, shock strut 100 is unacceptable for use with an aircraft tail skid that has no tire to absorb and mitigate the jouncing motion. Without a tire, the jouncing motion is directly transferred to the aircraft structure. Therefore, an improved shock strut similar to shock strut 100 is desired having a reduced or eliminated tendency to generate an oscillatory suspension load following a sudden impact.